Ultrasonic Pipe Testing Benefits

Ultrasound is a well proven and respected diagnostic tool routinely employed for weld and flaw detection in critical applications such as aviation, aerospace, military, marine, and nuclear power.  Developed in the 1950s, it has evolved significantly in terms of equipment capability, accuracy, the level of information provided, and applications.

Yet, while advancements in instrumentation have moved this technology into other areas such as manufacturing and quality control, its benefits to plant engineers and property owners as a diagnostic and predictive tool are still widely unknown and underutilized.

• Limitations Of Metallurgical Analysis

As a nondestructive method, UT offers obvious advantages over cutting out pipe for metallurgical inspection.  Removing a pipe sample requires interrupting service, draining down all or part of the system, hiring a mechanical contractor to cut out the pipe, welding in a spool piece replacement, re-filling the system, and then adding new chemical to raise treatment levels back up to their required levels.

Far more of a concern than the physical aspect of removing the pipe is the question of what to remove.  This question is most easily answered where a failure has occurred; that failure now defining the specific area of interest.  But for general inquiries, one needs special mechanical clairvoyance in selecting the one or two examples of pipe representative of the entire system.  Removing a sample of pipe not relative to the concern, and the entire effort becomes either worthless or even worse – misleading to the corrosion conditions present.

Most pipe is removed where easiest and most convenient, rather than where the most appropriate.  Smaller pipe is favored for the obvious reasons, which means a lab analysis result quite different from what exists at the larger diameter mains.  Data centers, critical operations, and almost all large commercial office building are today 24/7 operations, which entirely eliminates any possibility of determining the condition of any pipe unless it can be briefly isolated from the main.  For most properties, cutting out any any 24 in. or larger diameter piping for metallurgical inspection is totally impossible.

Basing capitol decisions on a piping evaluation providing limited information gained from one or two non-representative samples is therefore a risky gamble.

A further issue is the amount of information derived.  Tasked with identifying the underlying cause of a weld failure, pinhole at a copper domestic water line, a split weld seam, a thread or soldered joint failure, or to confirm or identify the ASTM specification of a section of questionable steel pipe – metallurgical testing excels as an investigative tool.  For the more common questions below, ultrasonic testing combined with thorough data analysis is far more useful.

• • • What is the corrosion rate?
    • Is corrosion activity higher than normal?
    • Is our chemical water treatment program effective?
    • Why are our strainers clogged with rust?
    • How long will this pipe last?
    • What and where are my system vulnerabilities?
    • Are additional failures likely?
    • How much pipe requires replacement?

A metallurgical report showing charts of the metal’s chemical composition, before and after photos of cleaned and dissected pipe, and microscope based images of internal grain structure often yield little useful information in answering any of the above questions.  An ultrasonic investigation, however, when performed properly and where the collected data is further analyzed into a professional quality report, will definitively provide such information and usually much more.

For one New York City consulting engineering firm, cutting out 50 samples of pipe from their client’s HVAC, plumbing, and fire systems for lab analysis still didn’t answer their questions.  After spending two months and $365,000 in total cost for a general contractor, mechanical contractor and laboratory service, or $7,300 per sample, photos of cleaned and polished inside pipe surfaces failed to provide any indication to their condition, corrosion rate, or remaining service life. 

Decisions were instead entirely based upon a hurriedly performed one day ultrasonic examination of 55 examples of pipe by CorrView at a cost of $4,000 – 98.9% less!

The tremendous benefit of a metallurgical lab examination is best reserved where failure has already occurred, to questions specifically related to the physical composition of the pipe, and to those examples of pipe first identified by an ultrasonic examination as having the need for a closer look.  Surprisingly common, this valuable benefit is lost when the failed and removed pipe section(s) have been thrown away or sold for scrap for a few dollars.

• The Many Advantages Of Ultrasound

Ultrasonic testing, on the other hand, is non-intrusive, accurate, reliable, safe to both building and inspection personnel, provides immediate results, requires no system shutdown, and is extremely cost effective.  Depending upon the measurement technique, degree of testing, data analysis method used, and the competence of those performing the inspection, ultrasound can produce a thorough assessment of building piping condition, provide direction for capital projects, or focus in on a specific area of concern.

Such advanced information is becoming more valuable to plant engineers as the former “run to failure” mode of operation moves toward one where all known vulnerabilities of an HVAC operation are known and monitored, and where long term planning has hopefully replaced unexpected failures and emergency repairs.  Establishing the condition of an aged piping system becomes especially important due to its critical function in any HVAC building environment, the usually massive undertaking in its replacement, and due to the wide variety of corrosion related problems which can potentially develop.

• Multiple Negative Influences

Generally unrecognized by building owners / operators, and even professional engineers tasked with designing such systems, the corrosion threat to most piping systems has increased dramatically over the past 25 years.  This is due to:

• • • Less effective chemical protection / lower treatment levels
    • Lower quality piping products – steel, galvanized steel, copper, cast iron, ductile iron, stainless steel
    • Greater vulnerability of all new pipe products to corrosion
    • Undersized thinner wall pipe / manufacturing to minimum ASTM specifications
    • Lower cost / lower quality foreign pipe products
    • Migration to thinner piping schedules and significantly lesser wall thickness
    • Use of ERW welded pipe rather than seamless
    • Thinner and lower quality galvanized steel coatings
    • “Green” design and operating demands favoring higher corrosion levels
    • Value engineering

• Thorough Testing Required

In order to provide the greatest degree of reliability, any evaluation method must address all the different sizes of pipe installed, the furthermost areas of the system, top and bottom areas, horizontal and vertical runs, as well as threaded, grooved, and welded pipe.  Since every piping system has its own vulnerabilities to corrosion, knowledge to such conditions are critical in order to select the appropriate and most information revealing test locations.  A piping evaluation, therefore, must address sufficient sections of pipe at its most vulnerable areas, as well as perform repetitive testing at each location – leaving ultrasound as the overwhelmingly preferred choice.

Taking multiple wall thickness readings at any pipe section not only identifies its current status, but more importantly, provides a virtual image of its interior wall thickness profile.  The more uniform the result, the more likely a mild and general corrosion condition exists.  Conversely, a wildly varying thickness profile will indicate not only a pitting condition, but also the high probability that even lower thickness values exist.

Below left, this profile of 12 wall thickness values from a section of 40 year old hot water heating pipe is extremely uniform.  Although showing some loss from original 0.280 in. schedule 40 specifications, this 6 in. pipe exists under a very low 0.4 MPY  corrosion rate and will provide virtually unlimited service life to the facility.  At the center, we show the results from a heavily deteriorated open condenser water system where deep pitting now defines its remaining service life.  Below right, severe pitting at a 4 in. schedule 10 galvanized steel pre-action fire sprinkler line has reduced wall thickness in some areas to near that of a common credit card.  With such higher deterioration present, a further statistical estimate of lowest potential wall thickness based upon its standard deviation mandates that even lower wall thickness exists in other areas of the system.

Typically, a highest to lowest range in wall thickness of 0.100 in. or more strongly suggests, by itself, a severe corrosion condition likely exceeding 10-15 MPY.  It also raises an even greater concern for those most vulnerable areas of the system.  For any piping system having a 0.100 in. random pitting condition, simple math alone would indicate the potential for leaks of failure at any threaded pipe of 2 in. and below having a beginning wall thickness of 0.082 in. or less.

• Determine Corrosion Rate

With the original pipe wall thickness and time in service known, calculations can be made regarding the approximate rate or mils per year (MPY) that the pipe has reached its current thickness level.  Even though the pipe is not likely to have corroded evenly over time, such corrosion rate estimates are generally accurate, and will fall within a certain range of values depending upon piping service.

A theoretical minimum acceptable wall thickness calculation, or an estimate of the lowest point the pipe can be allowed to safely operate, can also be made based upon known material strength, pipe diameter, operating pressure, thread loss, temperature, and corrosion factor.  This allows a further prediction of the remaining service life at the pipe according to the time it will take to deteriorate from its measured current wall thickness, at the current MPY rate of wall loss, to its minimum acceptable value.  From this point, a retirement date or remaining service estimate can be offered.

If sufficient pipe locations are tested, individual results can be grouped according to various criteria, and graphed to show any similarity or differences within the same piping service.  Such further analysis helps us to identify errors in the raw data, but most importantly will highlight any corrosion trends within the piping system.  Data analysis and trending may show, for example, a higher corrosion rate at the smaller low flow areas, greater losses at the warmer return side condenser water piping, or greater corrosion activity at pre-action fire sprinkler systems over their wet counterparts.

We provide a full explanation of our ultrasonic pipe testing format on this Internet site.

• Identify System Status

Overall, ultrasonic pipe testing offers tremendous benefits.  For many building operators, an ultrasonic report will very often provide the very first suggestion of a corrosion problem or concern – and provide the advance notice required to address it effectively.  Years of 0.3 MPY corrosion coupon results may, in reality, prove to be substantially higher at 5 MPY.

We outlined five objectives of our ultrasonic investigations and believe that the final reports accomplish those objectives listed below:

1. Accurately assess the current condition of the system in terms of remaining wall thickness, wall loss, corrosion rate, and estimated remaining service life.

2. Identify any system weakness, and immediate and near term threats of leak or failure.

3. Document specific findings and observations contributing to such abnormally higher corrosion activity.

4. Offer recommendations toward a workable, safe, and effective solution to the problem, if possible, and depending upon the remaining condition of the pipe.

5. Establish whether current planning to correct corroded piping conditions, such as might be planned through chemical cleaning, can be safely carried out.

Ultrasonic pipe testing can provide irrefutable evidence of a suspected corrosion problem, or document that a piping system has fulfilled its normal service life and is in need of replacement.  At the high costs associated with any capitol piping replacement, an ultrasound report will provide the hard documentation necessary to move the project both forward, as well as in the right direction.  Similarly, it can save money by confirming that suspected bad pipe is still suitable for decades of additional use, or limit repairs to specific areas.  For a building acquisition, ultrasound will show the true condition of all piping systems; exposing issues which will never be found in a handbook of service life estimates.

Where no problems exist, ultrasound will provide greater security, and most importantly, establish a solid baseline from which future and even more accurate and reliable estimates of corrosion rate and remaining pipe life can be made.

© Copyright 2024 – William P. Duncan, CorrView International, LLC